This invention relates to a device for precisely controlling the frequency of a local oscillator and more particularly to a device for controlling the frequency of a voltage control local oscillator by detecting the frequency of an output signal therefrom and thereby effecting voltage control by an output of said detection.
Where an attempt is made to receive FM broadcast waves of a given channel, the general practice is to operate a variable condenser in the case of a prior tuner so as to continuously vary the frequency of a local oscillator included in said tuner, thereby attaining tuning with the desired channel. Though this procedure can indeed realize tuning, yet it does not necessarily follow that an intermediate frequency obtained coincides with the prescribed intermediate frequency. Therefore, errors are likely to take place in the attempted tuning.
In general, the frequencies of broadcast waves of various channels are specified in units of a prescribed step or interval. In Japan, for example, the frequencies of the respective FM broadcast waves are specified in units of 100 kHz. Since, the frequency of FM broadcast waves of any desired channel consists of a known multiple of 100 kHz units allotted thereto, it is unnecessary to vary the frequency of a local oscillation as often as need arises. What is required is only to cause the local oscillator to oscillate exactly at the frequency allotted to any desired FM channel in order to obtain any desired intermediate frequency, and carry out reception without tuning errors.
It is accordingly the object of this invention to provide an oscillation frequency control device which enables a local oscillator included in a tuner to oscillate only at a frequency allotted to any desired channel.